1. Field of the Invention
This invention relates to the extrusion of plastic pipes and other profiles, and more particularly to a method of controlling a thickness dimension, such as wall thickness in a pipe profile, by measuring the thickness dimension and adjusting heating elements positioned at the die outlet in order to increase the temperature of the extruded material and thereby increase the thickness at a desired point on the article, and adjusting the heating elements in order to decrease the temperature and thereby decrease the thickness at a desired point on the article, to maintain a desired thickness profile in the extruded article.
2. Description of the Related Art
In the continuous extrusion of plastic profiles, such as pipes, the wall thickness of the extrusion can be controlled to a certain degree by raising or lowering the haul-off speed, whereby the extruded article is pulled away from the extrusion die after sufficient cooling has occurred to permit handling of the article without permanent deformation. That method of control affects the average thickness of the article, but it has no effect with regard to the correction of any eccentricity or non-uniformity that may exist in the thickness. In an extruded pipe such eccentricity would be manifested in an extruded pipe structure wherein certain circumferential portions of the pipe have different wall thicknesses from other circumferential portions.
In order to correct any eccentricity that may exist in an extruded pipe, the extrusion die must be reset in order to reposition the center mandrel with respect to the surrounding outer die structure in order to bring the axis of the mandrel into substantial concentricity with the surrounding outer die structure. To permit such repositioning, however, imposes significant structural complications upon the extrusion die structure, whether such correction is capable of being performed either manufally or automatically, such as by means of motors or the like.
The control of the haul-off rate, in addition to affecting the average pipe wall thickness, is monitored principally to control the minimum wall thickness so that it falls within the prescribed wall thickness tolerance. However, when controlling the extrusion process by that method, unnecessary and excessive material can be extruded if the inner and outer die faces are not concentric. Thus, in order to meet the minimum wall thickness standard, excessively thick portions of material exist at other parts of the pipe cross section, which unnecessarily increases the material costs for producing such pipes.
In connection with the mechanical centering of the parts of the extrusion die to improve concentricity, that particular process is cumbersome, often imprecise, and the use of motors and other automatic equipment introduces unnecessary complexity to the die structure, which results in increased die costs, as well as increased die maintenance resulting from wear of the moving parts, breakage of parts, and the like. One example of apparatus that has been devised to permit centering of a core with respect to an extruder head is illustrated and described in U.S. Pat. No. Re. 29,886, which issued Jan. 16, 1979, to Matthew R. Dembiak et al.
One form of apparatus for controlling the wall thickness of a pipe by controlling the haul-off speed is illustrated and described in U.S. Pat. No. 4,137,025, which issued Jan. 30, 1979, to Kenneth E. Graves et al.
It is an object of the present invention to overcome the shortcomings of the prior art devices and methods, and to provide a simple and efficient method of controlling the wall thickness distribution of an extruded article in order to maintain a desired thickness profile and without the need for mechanical shifting of parts of the extrusion die.